Optical touch system and image processing method thereof

ABSTRACT

An optical touch system includes an image sensor module and a processor. The image sensor module includes a plurality of image sensing elements that can be independently controlled to achieve different exposure times. The plurality of image sensor elements can produce a picture including a plurality of pixel groups. The processor is configured to extract an intensity value of each pixel group and to select a set of successive pixel groups as an object image according to the intensity values of the pixel groups.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based on, and claims priority from, TaiwanPatent Application Serial Number 100121547, filed on Jun. 21, 2011, thedisclosure of which is hereby incorporated by reference herein in itsentirety.

BACKGROUND

1. Technical Field

The present invention relates to an optical touch system and an imageprocessing method thereof.

2. Related Art

An optical touch system generally comprises an imaging device, anilluminating device, and a computing device for determining the touchposition of an object. Typically, the object may be a finger, a stylus,or the like. The imaging device is configured to view a touch zone abovea touch surface. The illuminating device is configured such that when anobject is in the touch zone, the illuminating device can make the objectgenerate an identifiable contrast image on a picture produced by theimaging device. The computing device is configured to calculate thecoordinates of the object according to the brightness variation of thepicture produced by the imaging device.

The optical touch system can be designed as a system in which the objectentering into the touch zone blocks the light projected from theilluminating device so as to form a dark area on the imaging device, oras a system in which the object reflects the light projected from theilluminating device to form a bright area. Regardless of which system isemployed, the current coordinate calculating method requires twopictures, a background picture and a picture taken when there is anobject in the touch zone. Normally, the background picture is generatedand stored before operation. The optical touch system can identify theregion having obviously different intensity by comparing the capturedpicture and the background picture. When the region having obviouslydifferent intensity is used as an image formed by an object, the opticalsystem can calculate the coordinates of the object according to somefeatures of the region having obviously different intensity.

In addition to the region having obviously different intensity, theintensity levels of a portion of the background area of the picture maybe changed due to the change of the light path or the manner of lightreflection of the touch surface caused by the object such that adifference may occur between a background area of the picture and thecorresponding portion of the background picture. Such difference maymake it impossible for the region having obviously different intensityto be correctly calculated or properly identified. As a result, thecoordinates of the object cannot be accurately calculated.

SUMMARY

One embodiment provides an image processing method and an optical touchsystem using the same. The image processing method can use a singlepicture to determine the coordinates of an object such that incorrectcoordinates will not be obtained when there is a difference between abackground area of a picture and the corresponding portion of abackground picture.

In one embodiment, an optical touch system comprises an image sensormodule and a processor. The image sensor module comprises a plurality ofimage sensing elements. The image sensing elements are configured to beindependently controlled to achieve different exposure times. Theplurality of image sensing elements may generate a picture comprising aplurality of pixel groups. The processor is configured to extract anintensity value representing each pixel group. The processor is alsoconfigured to select a portion of the pixel groups as an object imageaccording to the intensity values of the pixel groups.

In one embodiment, an image processing method of an optical touch systemcomprises obtaining a picture comprising a plurality of first pixelgroups, determining a plurality of first difference values eachdetermined by subtracting intensity values of two of the plurality offirst pixel groups, and selecting a set of successive pixel groups as anobject image according to the first difference values.

To better understand the above-described objectives, characteristics andadvantages of the present invention, embodiments, with reference to thedrawings, are provided for detailed explanations.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described according to the appended drawings inwhich:

FIG. 1 is a schematic view showing an optical touch system according toone embodiment of the present invention;

FIG. 2 is an illustration of a picture according to one embodiment ofthe present invention;

FIG. 3 schematically depicts an intensity fluctuating pattern accordingto one embodiment of the present invention;

FIG. 4 schematically depicts a portion of sensing elements and thecircuit of an image sensor module according to one embodiment of thepresent invention;

FIG. 5 is a flow chart related to an image processing method accordingto one embodiment of the present invention;

FIG. 6 schematically depicts an intensity fluctuating pattern accordingto another embodiment of the present invention;

FIG. 7 schematically depicts a difference fluctuating pattern accordingto one embodiment of the present invention;

FIG. 8 is a flow chart related to an image processing method accordingto another embodiment of the present invention;

FIG. 9 schematically depicts an intensity fluctuating pattern accordingto another embodiment of the present invention;

FIG. 10 schematically depicts a difference fluctuating pattern accordingto another embodiment of the present invention;

FIG. 11 is a flow chart related to an image processing method accordingto another embodiment of the present invention;

FIG. 12 schematically depicts an intensity fluctuating pattern accordingto another embodiment of the present invention;

FIG. 13 schematically depicts a difference fluctuating pattern accordingto another embodiment of the present invention;

FIG. 14 is a flow chart related to an image processing method accordingto another embodiment of the present invention;

FIG. 15 schematically depicts an intensity fluctuating pattern accordingto another embodiment of the present invention;

FIG. 16 schematically depicts a difference fluctuating pattern accordingto another embodiment of the present invention;

FIG. 17 schematically depicts an intensity fluctuating pattern accordingto another embodiment of the present invention; and

FIG. 18 schematically depicts a difference fluctuating pattern accordingto another embodiment of the present invention.

DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS

The following description is presented to enable any person skilled inthe art to make and use the disclosed embodiments, and is provided inthe context of a particular application and its requirements. Variousmodifications to the disclosed embodiments will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to other embodiments and applications without departing fromthe spirit and scope of the disclosed embodiments. Thus, the disclosedembodiments are not limited to the embodiments shown, but are to beaccorded the widest scope consistent with the principles and featuresdisclosed herein.

FIG. 1 is a schematic view showing an optical touch system 1 accordingto one embodiment of the present invention. One embodiment of thepresent invention is related to an image processing method that cancalculate the coordinate data using a single picture. The imageprocessing method is applicable to the optical touch system 1 shown inFIG. 1. Referring to FIG. 1, the optical touch system 1 comprises animage sensor module 11 and a processor 12. The processor 12 is coupledwith the image sensor module 11 to analyze an object image from thepicture generated by the image sensor module 11. The image sensor module11 is configured to monitor a touch area 13. The illuminating devices 14and 15 are disposed adjacent to the touch area 13 to provideillumination such that the object on the touch area 13 can generate anidentifiable image on a picture. The illuminating device 14 or 15 can bean active light source including, for example, a light tube, a pluralityof light emitting diodes, or a combination of a light emitting diode anda light guide member. The illuminating device 14 or 15 may also be apassive light source such as a mirror.

The image processing method of one embodiment of the present inventioncan be applied using an illumination-compensated picture whose imageintensity variation is compensated for and with a normally capturedpicture whose image intensity variation is not compensated for. Thedescription below begins by describing application of the imageprocessing method of one embodiment of the present invention to anillumination-compensated picture.

The method of compensating for image intensity variation can beimplemented as software or hardware means and is applicable to a pictureto increase the intensity of the portion that originally has lowerintensity and to reduce the intensity of the portion that originally hashigher intensity, so as to obtain a new picture with uniform intensity.For example, as shown in FIG. 2, the image sensor module 11 may producea picture 2. The picture 2 may comprise a plurality of pixel groups 21that may be arranged along a direction. Each pixel group 21 may comprisea plurality of pixels 211. In one embodiment, each pixel group 21 may bea pixel column, and the plurality of pixel groups 21 are arranged alonga row direction. In another embodiment, the pixel group 21 may be apixel row, and the plurality of pixel groups 21 are arranged along acolumn direction. The processor 12 is configured to extract an intensityvalue representing each pixel group 21 from the captured picture 2. Inone embodiment, the intensity value representing a pixel group 21 can bea sum of the intensity values of the pixels 211 of the pixel group 21.In another embodiment, the intensity value representing a pixel group 21can be an average of the intensity values of the pixels 211 of the pixelgroup 21. After the calculation of the intensity values of all pixelgroups 21 is completed, an intensity fluctuating pattern 3 as shown inFIG. 3 can be obtained.

As illustrated in FIG. 3, the intensity fluctuating pattern 3 exhibitssignificant variation. The intensity fluctuating pattern 3 may becompensated for in advance. In one embodiment, an adjustment value foreach pixel group 21 is determined. Next, each adjustment value ismultiplied by the intensity value of the corresponding pixel group 21 toobtain a new intensity fluctuating pattern 4 exhibiting less variation.In one embodiment, the adjusted intensity fluctuating pattern 4 can varywithin an intensity range 5.

The adjustment value is used to compensate for the variation of anintensity fluctuating pattern. The adjustment value can be determinedthrough many methods, one of which is illustratively demonstratedherein. In one embodiment, an adjustment value can be obtained by thefollowing steps: A background picture is generated using a fixedexposure time. Next, an intensity value (I_(n)) of each pixel group isdetermined. Finally, a ratio of a target intensity value (I_(T)) to theintensity value (I_(p)) for each pixel group is calculated, wherein theratio (I_(p)/I_(T)) can be used as an adjustment value.

The compensation for the intensity variation of the picture can beachieved through hardware. As illustrated in FIG. 4, the image sensormodule 11 comprises a plurality of image sensing elements 41 a and 41 beach comprising an electronic shutter 411 and a photo detector 412. Theelectronic shutter 411 is coupled with the photo detector 412 to controlthe exposure time of the photo detector 412. The photo detector 412generates charge in response to received light. The transistor 413controls the transferring of the charge from the photo detector 412 to afloating diffusion (FD) output node. The transistor 414 and the constantcurrent source 415 form as a source follower, which can amplify thephotovoltaic voltage produced by the photo detector 412. The transistor416 can be activated when the signal WL1 is at a high level, and at thismoment, data can be output to the bit line 417 that is coupled to areadout circuit 220. When signals RST1 and TG1 go high, the transistors418 and 413 are activated such that the voltage supply VDDAY can resetthe photo detector 412 to a photo-electric conversion initiation state.When the transistor 413 is turned on, the charge flows from thephoto-detector 412 to the FD output node. When the signal RST1 goeshigh, the voltage source VDDAY resets the FD output node.

Referring to FIG. 4, the electronic shutters 411 of the image sensingelements 41 a and 41 b are respectively coupled to different shuttercontrol lines 419 a and 419 b. As such, different signals AB1 and AB2can be applied to the electronic shutters 411 of the image sensingelements 41 a and 41 b to operate the image sensing elements 41 a and 41b for different exposure times such that the intensity values of thecorresponding pixels of a picture can be independently manipulated.During operation, the image sensing element 41 a or 41 b, which isexposed to stronger light intensity, is assigned shorter exposure timeand the image sensing element 41 a or 41 b which is exposed to weakerlight intensity is assigned longer exposure time. As a result, acaptured picture can exhibit a more uniform background intensity level.

The exposure time for controlling the electronic shutter 411 of eachimage sensing element 41 a or 41 b can be obtained using the followingmethod; however, the present invention is not limited to such method.The method obtains a background picture by a fixed exposure time. Next,the intensity value of each pixel of the background picture isextracted. Thereafter, the exposure time that is needed for operatingthe corresponding electronic shutter 411 of an image sensing element 41a or 41 b and can make the corresponding pixel achieve a targetintensity level is computed using the target intensity level and theintensity value of the pixel.

FIG. 5 is a flow chart related to an image processing method accordingto one embodiment of the present invention. Referring to FIG. 5, at StepS51, a picture is captured, wherein the picture comprises a plurality ofpixel groups that can be arranged along a direction. At Step S52, theintensity value I(p_(i)) of each pixel group of the picture is computedto obtain an intensity fluctuating pattern I(p) as shown in FIG. 6,where p_(i) represents the i-th pixel group. The intensity value of eachpixel group can be the sum of the intensities of pixels or the averageof the intensities of pixels of the pixel group. At Step S53, a targetintensity value (T1) is decided. In one embodiment, the target intensityvalue (T1) can be the average of the intensity fluctuating pattern I(p)or the intensity values I(p_(i)). In another embodiment, the targetintensity value (T1) can be a predetermined value. At Step S54, adifference fluctuating pattern D1(p) formed by a plurality of differencevalues D1(p _(i)) each calculated, as shown in equation (1), bysubtracting the target intensity value (T1) from the correspondingintensity value I(p_(i)) on the intensity fluctuating pattern I(p) asshown in FIG. 7 is obtained.

D1(p _(i))=I(p _(i))−T1  (1)

At Step S55, a threshold Th1 is used to determine a section R1 of theintensity fluctuating pattern I(p) that includes pixel groups havingratios of difference values D1(p _(i)) to the target intensity value T1less than or greater than the threshold Th1 as an object image.

FIG. 8 is a flow chart related to an image processing method accordingto another embodiment of the present invention. Referring to FIG. 8, asStep S81, a picture is obtained. The picture comprises a plurality ofpixel groups that may be arranged along a direction. At Step S82, theintensity value I(p_(i)) representing each pixel group (p_(i)) of thepicture is extracted to obtain an intensity fluctuating pattern I(p) asshown in FIG. 9, where p_(i) represents the i-th pixel group. Theintensity value representing each pixel group can be either the sum ofthe intensity values of pixels of the pixel group or an average of theintensity values of pixels of the pixel group. At Step S83, a differencefluctuating pattern ICD(p) as shown in FIG. 10 is determined. Thedifference fluctuating pattern ICD(p) has a plurality of differencevalues ICD(p_(i)) sequentially calculated along the arrangementdirection of the pixel groups and each ICD(p_(i)) determined bysubtracting the intensity values of two different pixel groups (p_(i)and p_(i+1)). At Step S84, on the difference fluctuating pattern ICD(p),the point P_(t1) whose difference value is less than a threshold Th2 andthe point P_(t2) whose difference value is greater than a threshold Th3are determined. One of the pixel groups corresponding to the pointP_(t1), is used as a left boundary pixel group, and one of the pixelgroups corresponding to the point P_(t2) is used as a right boundarypixel group. Accordingly, the section of the intensity fluctuatingpattern constituted by a set of successive pixel groups between the leftand right boundary pixel groups is obtained, and the section can then beselected as an object image.

FIG. 11 is a flow chart related to an image processing method accordingto another embodiment of the present invention. Referring to FIG. 11, atStep S111, a picture comprising a plurality of pixel groups arrangedalong a direction is generated. At Step S112, the intensity valueI(p_(i)) representing each pixel group (p_(i)) of the picture isextracted to obtain an intensity fluctuating pattern I(p) as shown inFIG. 12, where p_(i) represents the i-th pixel group. The intensityvalue representing each pixel group can be either the sum of theintensity values of pixels of the pixel group or the average of theintensity values of pixels of the pixel group. At Step S113, sequentialcalculation of the difference value ICD(p_(i)) between a pixel groupp_(i) and a pixel group p_(i) that are spaced at an interval of apredetermined number q is performed one after another along thearrangement of the pixel group p_(i), to the pixel groups of the pictureto obtain a difference fluctuating pattern ICD(p) as shown in FIG. 13.At Step S114, on the fluctuating pattern ICD(p), the point P_(o) whosedifference value is less than a threshold Th4 and the point P_(t4) whosedifference value is greater than a threshold Th5 are determined. One ofthe pixel groups corresponding to the point P_(o) is used as a leftboundary pixel group, and one of the pixel groups corresponding to thepoint P_(t4) is used as a right boundary pixel group. Accordingly, thesection constituted by a set of successive pixel groups between the leftand right boundary pixel group is obtained, and the section can beselected as an object image 121 as shown in FIG. 12.

In particular, compared with the method of calculating the differencevalue of two adjacent pixel groups (p_(i) and p_(i+1)), the method ofcalculating the difference value between a pixel group p_(i) and a pixelgroup p_(i+q+1) that are spaced at a predetermined number q can obtainlarger difference values at the edges of the object image 121. As shownin FIG. 12, the difference value between two adjacent points 1212 and1213 on the left side edge 1211 of the object image 121 is about 50,while the difference value between two separated points 1212 and 1213 isabout 90. Moreover, due to the existence of noise, the method ofcalculating the difference value between a pixel group p_(i) and a pixelgroup p_(i+q+1) that are spaced at a predetermined number q is noteasily affected by noise.

The predetermined number q can be a user set number, or a valuedetermined by the modulation transfer function (MTF) of the image sensormodule 11, wherein the MTF is a measure of the transfer of modulation(or contrast) from an object to an image. An MTF value can be a point ona MTF curve or an average of points on an MTF curve. From a backgroundpicture, a pixel group is selected and an initial intensity value M_(o)representing the pixel group is extracted. The following equation (2),an MTF value, and the initial intensity value M_(o) are used todetermine the to number of iterations needed when a last iterated resultis less than a predetermined value, wherein the number of iterations canbe used as the number q.

$\begin{matrix}{M_{i + 1} = {\frac{1 - {MTF}}{1 + {MTF}}M_{i}}} & (2)\end{matrix}$

The description below is related to an image processing method that isapplicable to a normally captured picture whose image intensityvariation is not compensated for.

FIG. 14 is a flow chart related to an image processing method accordingto another embodiment of the present invention. Referring to FIG. 14, atStep 141, a background picture comprising a plurality of pixel groupsarranged along a direction is generated. At Step S142, intensity valuesIB(p₁) of the pixel groups of the background picture are extracted toobtain an intensity fluctuating pattern IB(p) as shown in FIG. 15, wherep_(i) represents the i-th pixel group. The intensity value IB(p_(i)) ofthe pixel group can be the sum or the average of the intensity values ofthe pixels of the pixel group. At Step S143, the sequential calculationof the difference value BSD(p₁) between a pixel group p₁ and a pixelgroup p_(i+q+1) that are spaced at an interval of a predetermined numberq is performed one after another along the arrangement of the pixelgroup p_(i), to the pixel groups of the background picture to obtain adifference fluctuating pattern BSD(p) as shown in FIG. 16. In anotherembodiment, the sequential calculation of the difference value betweentwo adjacent pixel groups (p_(i) and p_(i+1)) is performed one afteranother along the arrangement of the pixel group p_(i), to the pixelgroups of the background picture to obtain a difference fluctuatingpattern BSD(p).

At Step S144, a minimum of the difference fluctuating pattern BSD(p) isthen determined as a threshold Th6, and a maximum of the differencefluctuating pattern BSD(p) is determined as a threshold Th7. A StepS145, a picture comprises a plurality of pixel groups arranged along adirection obtained. At Step S146, an intensity value I(p_(i))representing each pixel group of the picture is extracted to obtain anintensity fluctuating pattern I(p) as shown in FIG. 17, where p_(i)represents the i-th pixel group. The intensity value I(p_(i)) of thepixel group can be the sum or the average of the intensity values of thepixels of the pixel group. At Step S147, the sequential calculation ofthe difference value ISD(p_(i)) between a pixel group p_(i) and a pixelgroup p_(i+1) that are spaced at an interval of a predetermined number qis performed one after another along the arrangement of the pixel groupp_(i), to the pixel groups of the picture to obtain a differencefluctuating pattern ISD(p) as shown in FIG. 18. In another embodiment,the sequential calculation of the difference value between two adjacentpixel groups (p_(i) and p_(i+1)) is performed one after another alongthe arrangement of the pixel group p_(i), to the pixel groups of thepicture to obtain a difference fluctuating pattern ISD(p). At Step S418,on the difference fluctuating pattern ISD(p), the point 811 whosedifference value is less than the threshold Th6 and the point 812 whosedifference value is greater than a threshold Th7 are determined. One ofthe pixel groups corresponding to the point 811 is used as a leftboundary pixel group, and one of the pixel groups corresponding to thepoint 812 is used as a right boundary pixel group. Accordingly, thesection R2 constituted by a set of successive pixel groups between theleft and right boundary pixel groups can be obtained. The section R2 canthen be selected as an object image.

The data structures and code described in this detailed description aretypically stored on a non-transitory computer-readable storage medium,which may be any device or medium that can store code and/or data foruse by a computer system. The non-transitory computer-readable storagemedium includes, but is not limited to, volatile memory, non-volatilememory, magnetic and optical storage devices such as disk drives,magnetic tape, CDs (compact discs), DVDs (digital versatile discs ordigital video discs), or other media capable of storing code and/or datanow known or later developed.

The methods and processes described in the detailed description sectioncan be embodied as code and/or data, which can be stored in anon-transitory computer-readable storage medium as described above. Whena computer system reads and executes the code and/or data stored on thenon-transitory computer-readable storage medium, the computer systemperforms the methods and processes embodied as data structures and codeand stored within the non-transitory computer-readable storage medium.Furthermore, the methods and processes described below can be includedin hardware modules. For example, the hardware modules can include, butare not limited to, application-specific integrated circuit (ASIC)chips, field-programmable gate arrays (FPGAs), and otherprogrammable-logic devices now known or later developed. When thehardware modules are activated, the hardware modules perform the methodsand processes included within the hardware modules.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed embodiments.It is intended that the specification and examples be considered asexemplary only, with the true scope of the disclosure being indicated bythe following claims and their equivalent.

1. An optical touch system comprising: an image sensor module comprisinga plurality of image sensing elements configured to be independentlycontrolled to achieve different exposure times, the plurality of imagesensing elements generating a picture comprising a plurality of pixelgroups; and a processor configured to extract an intensity valuerepresenting each pixel group and select a portion of the pixel groupsas an object image according to the intensity values of the pixelgroups.
 2. The optical touch system of claim 1, wherein the picture isan illumination-compensated picture.
 3. The optical touch system ofclaim 1, wherein each pixel group comprises a plurality of pixels,wherein the intensity value representing each pixel group is a sum or anaverage of intensity values of the pixels of the pixel group.
 4. Theoptical touch system of claim 1, wherein each pixel group is a line ofpixels.
 5. The optical touch system of claim 1, wherein the processor isconfigured to subtract a threshold value from the intensity valuerepresenting each pixel group to obtain a plurality of difference valuesand to select the portion of the pixel groups according to ratios of thedifference values to the threshold value.
 6. The optical touch system ofclaim 1, wherein the portion of the pixel groups is greater or smallerthan a threshold value.
 7. The optical touch system of claim 5, whereinthe threshold value is an average of the intensity values representingthe pixel groups or a predetermined value.
 8. The optical touch systemof claim 1, wherein the image sensor module is configured to generate abackground picture using a fixed exposure time and use an intensityvalue of the background picture corresponding to each pixel group toadjust the exposure times of corresponding image sensing elements. 9.The optical touch system of claim 1, wherein the image sensor module isconfigured to adjust the exposure time for each image sensing element toallow the image sensor module to generate a new background picturecomprising a plurality of pixel groups having intensity values within anintensity range.
 10. An image processing method of an optical touchsystem, comprising the steps of: obtaining a picture comprising aplurality of first pixel groups; is determining a plurality of firstdifference values each determined by subtracting intensity values of twoof the plurality of first pixel groups; and selecting a set ofsuccessive pixel groups as an object image according to the firstdifference values.
 11. The method of claim 10, wherein the picture is anillumination-compensated picture.
 12. The method of claim 10, whereinthe step of determining a plurality of first difference values comprisesa step of determining a first difference value between the intensityvalues of each two adjacent ones of the plurality of first pixel groups.13. The method of claim 10, wherein the step of determining a pluralityof first difference values comprises a step of determining a firstdifference value between the intensity values of a pair of first pixelgroups separated by an interval of a predetermined number.
 14. Themethod of claim 13, wherein the predetermined number is determined bythe steps of: obtaining an MTF value of an image sensor; selecting theintensity value of one of the first pixel groups as an initial intensityvalue; using the following equation, the MTF value and the initialintensity value to determine a number of iterations needed when aniterated result is less than a predetermined value:$M_{i + 1} = {\frac{1 - {MTF}}{1 + {MTF}}M_{i}}$ where M_(i) representsan intensity value on the i-th iteration and K_(i+1) represents aniterated result on the i-th iteration; and selecting the number ofiterations as the predetermined number.
 15. The method of claim 10,further comprising the steps of: determining, from the plurality offirst pixel groups, a first boundary pixel group corresponding to thefirst difference value less than a first threshold value; determining,from the plurality of first pixel groups, a second boundary pixel groupcorresponding to the first difference value greater than a secondthreshold value; and selecting a plurality of ones of the pixel groupsbetween the first and second boundary pixel groups as the object image.16. The method of claim 15, further comprising the steps of: obtaining abackground picture that comprises a plurality of second pixel groups;determining a plurality of second difference values each determined bysubtracting intensity values of two of the plurality of second pixelgroups; selecting a minimum from the second difference values as thefirst threshold value; and selecting a maximum from the seconddifference values as the second threshold value.
 17. The method of claim10, wherein each first pixel group comprises a plurality of pixels, andthe intensity value representing each pixel group is a sum of intensityvalues of the pixels of the pixel group.
 18. The method of claim 10,wherein the pixel group is a line of pixels.